I've already beaten all 101 stages with both characters and the fastest time was just under 2 hours. The last 20 are nerve-wracking. I'll give my tactics for the most difficult creatures, so I don't waste too much space. I'll try to include one for both characters, but I'll be leaning more towards Dante, as he is the most tactically constructed one. (I don't really remember most of their names, so screw it.)
Lizard men (With plant attached)- The single most irritating creature in the game, these things are flying around constantly. Pin them down. Use Split and Streak/Helm Breaker and Stinger and keep knocking them on their asses, shotgunning and charge shotting them when you can, so when the plant dies, you can get rid of one quickly without taking damage.
Ice Lizards- They hate Gilgamesh. It phases them better and does the most damage. As for Nero, as long you can rev up your sword during strikes, he'll be effective too, and keep using his charge shots, they're really good at knocking these things down so you can reduce the amount attacking you at once. Funny side-note, during most of their ice moves they jump straight up into the air, and if you jump off of them with enemy step, they just fall on their faces. That happens with a lot of enemies, but it's easiest with them.
Lightning Lizards (Blitz)- Nero shines with these things. Keep charging shots and blasting him until his shield wears off. Them attack him in DT if you can. When his health reaches about 35%, grab him and Nero will beat the living daylights out of him for you. Charge a shot while he's doing that and let it go when Blitz tries to go red. Then that's it for him, and SSS for you. As for Dante, scrap that plan. Just get him to go red using the shotgun against his shields and survive with trickster. Killing him isn't worth it.
White Armor (Spear chuckers)- Okay, these things have the easiest timings. When they parry you as Nero, time your dodge to Table hop behind them. Not too difficult. For Dante, a strategy I find really useful is after the parry, do a short flip straight back locked on so you dodge the shield but are in range for the spear attack. Then time a royal release so that not only do you damage them, but you also fly through surrounding enemies (since these guys like to do that) and you also end up behind them. Releasing on the shield attack will not damage them or knock them into the air, so dodging the first attack is much more effective.
Gold Armor (Swordsman)- The main reason these guys are trouble is they choreograph the moves of the white armors. Alone they are simple though, so try to ignore them and take out the whites first. Their weakness is their shields. It only takes 3 Devil Busters to break them. For Dante, switch to gunslinger and use twosome time and Honeycomb fire when they block. Gun Stinger with the Shotgun works well, but leaves you open, so if you didn't break the shield, you'll get hit. Also keep in mind Yamato can't be parried. It goes straight through that stuff and does damage, particularly to shields.
Another note for all armors: Charge shots are effective for knocking them out of the air. So if you're bothered by them poking and running, charge up and teach them the consequences of range spamming.
I'll just say one more thing and end this obnoxiously long post (sorry guys). The person best suited for fighting Dante is NOT Dante. Nero can act on Dante's tick, which is shooting your bullets out of the air. When he does that, snatch him up and pound him. And STAY AWAY FROM DANTE IN ROYALGUARD! Timing is not an issue for the computer, so don't rush him. Just snatch and grab as Nero. As Dante, dash toward him so you get a jump off of him. Since this counts as an attack, he'll counter with a release and switch styles, but you won't get hurt. The rest is up to you. Good luck.
I hope I didn't just repeat other people's words and also hope somebody finds this useful. Peace, my brethren.